This invention relates to a fuel injection apparatus for injecting fuel into an internal combustion engine, especially a diesel engine.
In a prior art fuel injection apparatus of this type, fuel under high pressure supplied from a fuel supply pump is further pressurized in a fuel injection pump chamber by a plunger reciprocated in synchronism with an engine, and is sprayed from an injection nozzle into engine cylinders.
In injecting fuel into the engine, it is necessary to control fuel injection timing and fuel injection quantity according to the operational conditions of the engine. For such control, there has conventionally been developed control means using a solenoid valve, as stated in Japanese Patent Disclosure No. 50726/79. In this control means, the fuel injection quantity and fuel injection timing are controlled in accordance with electromagnetic pulses generated from the solenoid valve according to the operational conditions of the engine. However, since high pressure from the fuel is directly applied to the solenoid valve which has limited ability to withstand pressure so the prior art fuel injection apparatus cannot be used for injection under high pressure. To cope with this, a restrictor may be formed in a passage connecting the fuel injection pump chamber and the solenoid valve to lower the fuel pressure. The use of the restrictor would, however, cause gradual reduction of fuel quantity at the end of injection. Namely, it would retard the cut-off period of injection, and require a longer injection period for injecting the required quantity of fuel.
Since the fuel injection quantity depends only on the period during which the solenoid valve is closed, that is, on the injection period, the solenoid valve must be opened and closed in a very short time. Thus, the solenoid valve requires high responsiveness as well as high machining accuracy for the prevention of variation in injection quantity. These requirements are impractical.